This article, hoping to unfold the complicate picture of the Church and State relation, analyze and generalize the modern Vietnamese Buddhism and the process of its political participation. After WWII, the North and South Vietnams were separated, although their governments both declared Freedom of religion, the governors still manipulated, utilized or quelled the religion or their organizations repeatedly. The important connection between the Buddhism and the social order and the legitimatization of dominance in Vietnam can be reflected from the tensions between the Church and the State during the first Vietnamese Republic, to the Buddhist antiwar movement and finally to the whole control and use of religion for political propaganda by the Vietnamese Communist Party since the 1975 Reunion. By continual suppressions on Buddhists, the Diem Government not only cause the Buddhist protestation which lead to the coup d’état but also make the successive governments face the Buddhists’ political demands such as cease-fire, constitutional amendment and anti-corruption. Besides, an anti-American trend even emerged after the Communist Party's interpretation on these demands, which also complicated the reunion war. From “meditation practice” to “engaged Buddhist”, “sacred” to “trivialization”, followers of the United Buddhist Church, with their engaged and active attitude, use the traditional religious momentum to counteract the regimes and to facilitate the political reforms. And this reveals that the religion is the creation which performs human being’s value. Keywords: Vietnam, Buddhism and politics, United Buddhist Church, freedom of religion